How to let a tested class find a test assembly class via reflection?

I have a static method that uses reflection to find a class using reflection based on its name (qualified with namespace). The name is sent as a parameter to the method.

In essence, it looks like this (simplified):

public static Type GetClass(string qualifiedName)
{
    return Type.GetType(qualifiedName);
}

Now, to test this in an MSTest unit test, I want to let it look for a test class that doesn't exist in the tested assembly, but is declared in the unit test assembly. But when I pass the qualified name of my test class to GetClass, the call to Type.GetType returns null.

I assume the tested assembly's reflection system is unable to see stuff in the unit test assembly.

What do I need to do to ge this to work?